The vowels ए (ē) and ओ (ō) of Sanskrit, and therefore also of Hindi and other North Indian languages that have sprung from Sanskrit, are always long. They do not have corresponding short forms, such as इ (i) and उ (u) have the corresponding forms ई (ī) and ऊ (ū). This is because they do not actually reflect the simple vowels e and o of other Indo-European languages, but diphthongs ending in –i and –u respectively. ए developed from the diphthongs ai, ei and oi, and ओ developed from au, eu and ou. The reason behind the the lack of a short form of the same vowel in a language that does usually oppose pairs of long and short vowels is therefore one of etymology. It reflects how the language has grown over time, following, just as a living organism, internal laws and a will, one might say, of its own. So, while users of Sanskrit had long forgotten that their vowels ए and ओ once consisted of two short sounds, the phonetical inventory of their language had not. It retained in its DNA the traces of the provenance of those two sounds from diphthongs, which, consisting of two short vowels belonging to one syllable, are equivalent in length to long vowels.
This development becomes obvious, when one compares Sanskrit words containing one of the vowels in question with cognates in ancient Greek, which very faithfully represents diphthongs inherited from the Indo-European mother-language. The perfect οἶδα (oida) is (except that Greek lost the sound v) exactly corresponding to the Sanskrit perfect वेद (vēda), the Greek word for the colour white, λευκος (leukos), is etymologically identical with Sanskrit रोचः (rōcaḥ, Sanskrit frequently turns and original l into and r), which means bright or radiant. Also, the Latin word lūcus (loucos in Early Latin), meaning a glade, a clearing in a wood, is the same as Sanskrit लोकः (lōkaḥ), which means the world.
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